Abstract: Radical-ion-pair reactions were recently shown to manifest a host ofnon-trivial quantum effects accounted for by quantum measurement theory. Analternative approach purporting to describe the fundamental quantum dynamics ofspin-selective radical-ion pair reactions was introduced most recently,bringing to three the competing theories, including the one traditionally usedin spin chemistry. We here consider entropy as a fundamental concept enabling acomparison of the predictions of these theories against what is physicallyacceptable on quite general grounds.